Using new lines of transgenic (TG) mice, the investigators will determine the relative role of troponin I (TnI) and phospholamban (PLB) phosphorylation in the enhanced rate of relaxation during beta-adrenergic stimulation in control and acidic conditions. The investigator will also determine the impact of different levels of PLB phosphorylation on the regulation of Ca2+ homeostasis and contractility in the heart. New TG mouse lines will be produced by cross-breeding existing lines of mice: PLB deficient mice, TG mice that express mutated PLB at phosphorylation sites for protein kinase A, TG mice in which cardiac TnI (cTnI) has been replaced by slow skeletal TnI (ssTnI), and control mice that express normal levels of PLB and cTnI. Experiments will be done in the following mice: 1) deficient in PLB and expressing either native cTnI or ssTnI in place of cTnI; 2) expressing mutated PLB and either expressing native cTnI or ssTnI in place of cTnI; and 3) mice that express normal levels of PLB and express either native cTnI or have cTnI replaced by ssTnI. These studies will be performed at several levels of organization: whole heart, skinned fiber bundles from papillary muscles, as well as isolated single myocytes in the presence of absence of different concentrations of the beta-adrenergic agonist, isoproterenol, at pH 7.4 and 6.8. Using these new TG mice the investigator will test the hypothesis that both cTnI and PLB phosphorylation contribute to the enhanced relaxation during beta-adrenergic stimulation. Their relative contribution to the enhanced relaxation during different states of beta-adrenergic stimulation in physiological and pathological conditions will be assessed.